1. Field of the Invention
The present invention relates to a heater control apparatus for controlling energization of a heater which activates a gas detection element of a gas sensor.
2. Description of the Related Art
A gas sensor has been known which includes a gas detection element having at least one cell composed of a solid electrolyte member and a pair of electrodes, and which detects the concentration of a specific gas such as oxygen. When the temperature of the gas detection element rises, the gas detection element becomes active. Further, the solid electrolyte member exhibits good oxygen-ion conductivity, whereby detection of the concentration of the specific gas becomes possible. The gas detection element is heated by the heat of exhaust gas discharged from an internal combustion engine. Meanwhile, the gas sensor has a heater in order to activate the gas detection element quickly and secure stability of operation by maintaining the gas detection element at a fixed temperature. In general, a control apparatus for the gas sensor controls energization of the heater through PWM (pulse width modulation) control.
The control apparatus for the gas sensor detects the temperature of the gas detection element by detecting the resistance (impedance) thereof while detecting the concentration of the specific gas using the gas detection element. The control apparatus calculates a duty ratio used in the PWM control based on the detected element temperature, and controls energization of the heater to thereby maintain the temperature of the gas detection element constant. If the timing of on/off switching (starting/stopping) of the energization of the heater by the PWM control overlaps with the timing (period) of detection of the element resistance, a signal representing the detected element resistance may contain noise. A control apparatus is known which has solved such a problem by controlling the timing of on/off switching by the PWM control and the timing of detecting the element resistance such that they do not overlap with each other (see, for example, Patent Document 1).
[Patent Document 1] Japanese Patent No. 4241737
3. Problems to be Solved by the Invention
However, in the case of the control apparatus disclosed in Patent Document 1, when the timing of detecting the element resistance and the timing of on/off switching by the PWM control overlap with each other, the timing of detecting the element resistance is shifted or the period of the PWM control is shifted. In the case where the timing of detecting the element resistance is shifted, the control apparatus becomes unable to perform various processes at predetermined intervals and at predetermined timings in order to detect the element resistance. Since the control apparatus must adjust for the various processes in order to cope with changes in the processing timings, a problem arises in that the control becomes complex. Also, in the case where the period of the PWM control is shifted, since the control apparatus must calculate the duty ratio again in order to maintain the element temperature constant, a problem arises in that the control becomes complex.